Conventionally, decanter centrifuges comprise an elongated bowl, tapered at one end, and mounted for rotation about an axis. Coaxially mounted within the bowl is a helical screw conveyor which is adapted to rotate at a speed slightly different from that of the speed of the bowl. Such centrifuges are capable of continuously receiving feed material in the bowl and rapidly separating the feed material into layers of light and heavy phase materials, which are discharged separately from the bowl. It is the function of the screw conveyor to convey the outer layer of conveyable heavy phase material, usually solids or semi-solids, to a discharge outlet therefor, usually located in a tapered end portion of the bowl.
Effective and efficient centrifugal separation requires that the light phase material, usually liquid, be discharged through an outlet with the discharge containing little or no heavy phase material. In addition, the heavy phase material should contain only a small amount of light phase material. For example, if the light phase material is water and the heavy phase material comprises soft solids, it is preferred that clear water and relatively drier solids be separately discharged. This type of decanter centrifuge is disclosed in U.S. Pat. No. 3,795,361 and is hereby incorporated by reference.
In order to maintain an annular layer of light and heavy phase materials within the bowl, a dam structure is incorporated within the light phase material discharge outlet. The dam structure includes a weir surface which determines the annular depth or pond/pool level of the light and heavy phase feed materials within the bowl. In order to properly operate the decanter centrifuge, the pond level must be precisely set, especially when operating at neutral or negative dam settings, that is when the pond surface is radially even with or inward of the heavy phase discharge weir. Unfortunately, precise dam settings to achieve this pond level cannot be designed prior to installation of the centrifuge since dam settings depend upon the nature and characteristics of the feed mixture especially the separated solids. Moreover, the exact pond level is sensitive to the height of the liquid crest as it flows over the weir surface of the dam. This height of the crest must be considered in determining the depth of the pond.
The conventional approach is to adjust the dam settings upon installation. This is ordinarily accomplished by taking several different sized annular plate dams to the site, and exchanging them on a trial and error basis until an optimal pond level setting has been achieved. Obviously, this wastes an excessive amount of time. In addition, this process requires manufacturing several dam plates, yet only one dam plate is finally installed. Since this approach involves the transportation of ultimately superfluous parts, excessive installation time, and the manufacturing of ultimately unused parts, this conventional method is obviously inefficient and expensive.
As stated above, a factor which must be taken into consideration when setting the pond level is cresting. The height of the crest of the feed material above the weir surface varies as a direct function of the feed rate into the decanter centrifuge. When feed material is fed into the centrifuge at a relatively higher rate, a higher crest results, and hence a higher pond level. Since the feed rate to a decanter centrifuge is ordinarily not constant, it is desirable that the weir surface and dam structure be designed to minimize changes in crest height in response to changes in the feed rate into the centrifuge. In other words, it is desirable to have a weir geometry for which the height of the crest is relatively insensitive to the feed rate.
The present invention is directed to a device for adjusting the weir surface of the light phase materials dam. In particular, the interest is in adjusting the cresting height of the light phase materials which flow over the weir surface. To this end, a pair of annular plates are placed in coaxial parallel juxtaposition at the light phase materials discharge outlet of the centrifuge. Each of the plates includes a plurality of notches defining generally arcuate openings in the plate, the openings being located on the inner periphery thereof. The light phase materials are discharged through the arcuate openings. In the preferred embodiment, the plates have an identical number of arcuate openings positioned and spaced at the same angular location. Each of the arcuate openings is of an identical arcuate width, which defines the weir surface length, and a substantially identical depth positioned at a predetermined radial distance from the axis of rotation of the bowl. One annular plate is rotatably or angularly adjustable with respect to the other such that the arcuate width of the arcuate openings may be increased or decreased by increasing or decreasing the amount of overlap of the openings. Therefore, by rotating the plates relative to each other, the weir surface length can be increased or decreased, and, consequently, the crest height and pond level can be correspondingly changed. Moreover, this plate geometry is relatively insensitive to changes in feed rate of the feed material into the centrifuge.